Acoustic Foam

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Peter Kowalczyk

Freshman
Apr 12, 2013
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Truckee, CA
I'm looking for a source for absorptive acoustic foam in small bulk qtys (~50- 200 sq ft). IIRC, this has been discussed before, but my search of the forums results in lots of posts about grille foam, but few about acoustic absorption. Must be flame-retartdant. This source looks appropriate: Acoustical Foam - Soundproofing, Sound Insulation & Control Foam, but I'm wondering what others may be available.

And some questions to guide my search:

1) Is it a safe rule of thumb to say that the total RT60 reduction at any given frequency is correlated to the % coverage, while the absorption at low frequencies is correlated with foam thickness? Or conversely, could one trade off thickness for coverage (thicker foam, less coverage) and get similar results?

2) I see some materials specified with an absorption coefficient GREATER than 1 at certain (higher) frequencies, which seems impossible - how can a material absorb more than 100% of incident acoustic energy? What does this imply?
 
Re: Acoustic Foam

Peter Kowalczyk said:
I'm looking for a source for absorptive acoustic foam in small bulk qtys (~50- 200 sq ft). IIRC, this has been discussed before, but my search of the forums results in lots of posts about grille foam, but few about acoustic absorption. Must be flame-retartdant. This source looks appropriate: Acoustical Foam - Soundproofing, Sound Insulation & Control Foam, but I'm wondering what others may be available.

And some questions to guide my search:

1) Is it a safe rule of thumb to say that the total RT60 reduction at any given frequency is correlated to the % coverage, while the absorption at low frequencies is correlated with foam thickness? Or conversely, could one trade off thickness for coverage (thicker foam, less coverage) and get similar results?

2) I see some materials specified with an absorption coefficient GREATER than 1 at certain (higher) frequencies, which seems impossible - how can a material absorb more than 100% of incident acoustic energy? What does this imply?
Click to expand...
As with most things-there is not a simple answer.

Yes thickness matters-especially on the low freq side of things. But you also need total area and non flat surfaces (that can be reflective at high freq).

In terms of foam V shapes have better absorption than pyramid shapes.

Yes as a general rule the greater the total coverage the lower the RT60 at any freq.

If you go with thicker foam (and less of it) then you will have more reflections at the higher freq. However at lower freq you could space the panels apart with very little loss in effectiveness. In other words you need different approaches for low and high freq.-or what works for one does not apply to the other.

There are a couple of factors that can cause a panel to have a rating greater than 1. It all comes down to the tests and how they are doing. When doing the test the panel is assumed to have a surface area equal to the square area of the face of the panel. But when a single panel is put into a test chamber there is more "area" exposed than the simple measurements would imply. This is called "edge effect" and the edges of the panel will provide extra "area" to the reflections in the test chamber.

It is not actually absorbing "more than everything", just the way the tests reflect "the numbers".
 
Re: Acoustic Foam

You can find surplus/ "B stock"/slightly damaged etc foam on EBAY for some pretty good prices.

But it is not a "reliable" source. By that I mean you may not be able to buy more later on. So you need to get what you need at one time.
 
Re: Acoustic Foam

Thanks folks.

It makes sense that the actual surface area of a piece of textured foam (wedge, eggcrate, etc) is greater than it's footprint. If the absorption coefficient is calculated by normalizing to footprint area rather than actual surface area, I can see how values >1 could exist.

Of course, I'm getting ahead of myself - the contractors are still busy working hard to make every surface in this cafe reflective (glass, sheet metal, sheetrock and exposed brick abound). While I expect some treatment will be beneficial (necessary?), I'll wait to see how the room sounds (Reverberation as a function of frequency) before specifying the details. Too much LF -> a few thicker panels. Too much HF, many thinner panels.
 
Re: Acoustic Foam

For such installations be cautious about cheap wire cut general purpose foam that may not be fire resistant.

Convolved foam (with curved 3D patterns) is more expensive to tool up but can work better.

Professional foam will publish absorption numbers vs frequency so figure out what you need and let you fingers do some walking (math) before ordering foam.

Have fun..

JR
 
Re: Acoustic Foam

Consider that 'foam' may not be the most effective material and that how and where you apply any absorption can be as critical as the materials used. Also realize the potential of 'diminishing returns' where a large amount of absorption already present can require much larger changes for the same change in results than a room with minimal absorption to start with.

It should also be noted that few spaces exhibit actual statisitical reverberant fields where classic calculations would directly apply, most rooms are much more complex spaces which leads to how and where any absorption is applied being more relevant.
 
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